Assembly x86 reversing sentence program - string

Question

I am trying to make a program where the user have to enter an input, for example: Hello World and get an output: 'DLROw OLLEh'. Here is my program
org 100h
include emu8086.inc
.DATA
STR1 DB 0DH, 0AH, 'Input: $'
STR2 DB 0DH, 0AH, 'Output: $'
Nl DB 0Dh, 0Ah,'$'
.CODE
START:
MOV AX, #DATA
MOV DS, AX
DISP:
LEA DX,STR1
MOV AH,09H
INT 21H
MOV CL,00
MOV AH,01H
READ:
INT 21H
MOV BL, AL
PUSH BX
INC CX
CMP AL, 0DH
JZ DISPLAY
CMP AL, 'A' ; < then A
JB NotALetter
CMP AL, 'Z' ; > then Z
JA AGAIN ; repeat again
JMP CONTINUE1
AGAIN:
CMP AL, 'a' ; < then a
JB NotALetter
CMP AL, 'z' ; > then z
JA NotALetter
CONTINUE1:
JMP READ
DISPLAY:
LEA DX, STR2
MOV AH, 09h
INT 21H
LEA DX, NL
MOV AH, 09h
INT 21h
POP BX ; pop enter key
ANS:
MOV AH, 02h
POP BX ; pop the character
CMP BL, 'a' ; check if its in upper case
JB toLower ; if yes then jmp to toLower
SUB BL, 32 ; if not in upper case then convert to upper case
JMP CONTINUE2
toLower:
ADD BL, 32 ; convert to lower case
CMP BL, 20h
;SUB BL, 32
CONTINUE2:
MOV DL, BL
INT 21H
LOOP ANS
JMP EXIT ; if everything is fine jmp to exit
NotALetter:
printn
print "The input character is not a letter."
EXIT:
hlt
.EXIT
END START
I can enter any input but as soon as I enter any symbol, I am getting a message that this is a symbol, then program ends whereas I want to get the same output but still allow to enter a space character. I am really new in Assembly and moreover while I was trying to figure everything out I got even more lost.
If I comment out JB NotALetter and JA NotALetter, my space character becomes # probably because I am adding 20 to the ASCII hex number. Can someone please help to figure out this problem?

Answer 1

I can enter any input but as soon as I enter any symbol, I am getting
a message that this is a symbol, then program ends whereas I want to
get the same output but still allow to enter a space character.
As OP wants to capture the space without messing with symbol message. This can be achieved with the following:
In the READ label after you compare for enter key add this:
CMP AL, ' ' ; compare for space
JZ CONTINUE1
And in the ANS label after you pop bx add this:
CMP BL, ' ' ; if equal to space
JZ CONTINUE2 ; then print it by going to CONTINUE2 label

Answer 2

Just add an extra comparison to your toLower method as follows:
toLower:
CMP BL, 'A'
JL CONTINUE2
ADD BL, 32 ; convert to lower case
Complete code:
org 100h
include emu8086.inc
.DATA
STR1 DB 0DH, 0AH, 'Input: $'
STR2 DB 0DH, 0AH, 'Output: $'
Nl DB 0Dh, 0Ah,'$'
.CODE
START:
MOV AX, #DATA
MOV DS, AX
DISP:
LEA DX,STR1
MOV AH,09H
INT 21H
MOV CL,00
MOV AH,01H
READ:
INT 21H
MOV BL, AL
PUSH BX
INC CX
CMP AL, 0DH
JZ DISPLAY
CMP AL, 'A' ; < then A
JB CONTINUE1
CMP AL, 'Z' ; > then Z
JA AGAIN ; repeat again
JMP CONTINUE1
AGAIN:
CMP AL, 'a' ; < then a
JB CONTINUE1
CMP AL, 'z' ; > then z
JA CONTINUE1
CONTINUE1:
JMP READ
DISPLAY:
LEA DX, STR2
MOV AH, 09h
INT 21H
LEA DX, NL
MOV AH, 09h
INT 21h
POP BX ; pop enter key
ANS:
MOV AH, 02h
POP BX ; pop the character
CMP BL, 'a' ; check if its in upper case
JB toLower ; if yes then jmp to toLower
SUB BL, 32 ; if not in upper case then convert to upper case
JMP CONTINUE2
toLower:
CMP BL, 'A'
JL CONTINUE2
ADD BL, 32 ; convert to lower case
CONTINUE2:
MOV DL, BL
INT 21H
LOOP ANS
JMP EXIT ; if everything is fine jmp to exit
;NotALetter:
; printn
; print "The input character is not a letter."
EXIT:
hlt
.EXIT
END START
Input Hello World,
Output DLROw OLLEh
Also, you don't really need NotALetter method as you can notice, I just commented out.